This invention relates to a new and improved high speed gated peak detector apparatus for detecting and holding the peak amplitude value of a high frequency signal within a preselected time period.
Many signal processing applications have a requirement for peak amplitude detection of RF and pulse waveforms. In the non-destructive ultrasonic inspection and testing of parts, for example, the peak amplitude of a signal reflected from a flaw in the part is proportional to the size of the flaw, and the higher the frequency of the interrogating signal, the smaller the flaw size which can be detected. Sophisticated inspection systems are known which are capable of producing images of flaws. One such system employs a high frequency focused beam scanning acoustic microscope (SAM) and a computer for processing reflected flaw signals to produce enhanced images. Although scanning acoustic microscopes are capable of operating at frequencies of the order of 50 Mhz, the effectiveness of scanning acoustic imaging systems has been limited by the lack of a high speed gated peak detector. There are no available gated peak detectors which operate effectively above about 20 Mhz. Moreover, known gated peak detectors require a number of pulse repetitions (anywhere from 2 to 12, for example, depending upon frequency) for their output to settle to within 5% of the peak value. This necessitates using multiple pulses of a lower than desirable frequency and pulse repetition rate, and significantly increases the scanning and flaw acquisition time.
One approach which has been employed to avoid the frequency limitations of gated peak detectors has been to use a device such as a waveform recorder to increase the waveform time scale and decrease the frequency sufficiently (by a factor of 500, for example) to enable the peak detector to follow the waveform. Typically, samples of the waveform are digitized, stored in a memory, and clocked out at a fixed rate. For a 1 microsecond per word clock rate and a memory having a capacity of 1024 words, the cycle time for full memory readout is approxiately 1 millisecond, which sets the system's maximum repetition rate at 1000 pulses per second. Not only does this significantly increase the system scanning and defect acquisition times, such devices add unnecessary expense and complexity to the system.
In addition to their limited frequency and pulse repetition rate, other disadvantages of known gated peak detectors include their limited dynamic range (typically 20 dB), and their inaccuracy in measuring the peak amplitude of a single pulse or an asymmetrical waveform.
It is desirable to provide high speed gated peak detectors which avoid these and other problems of known gated peak detectors, and it is to this end that the present invention is directed.